Add documentation to debug missing position crash

Previously we just print the stacktrace, which is not
helpful for complex projects, as the console will be flooded
with other noisy messages.

Author: liufengyun

compiler/src/dotty/tools/dotc/ast/Positioned.scala

@@ -24,13 +24,7 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Pro
   def uniqueId: Int = myUniqueId
 
   def uniqueId_=(id: Int): Unit = {
-    if (Positioned.debugId == id) {
-      def printTrace() = {
-        val stack = Thread.currentThread().getStackTrace().map(">   " + _)
-        System.err.println(stack.mkString(s"> Debug tree (id=${Positioned.debugId}) creation \n> $this\n", "\n", "\n"))
-      }
-      printTrace()
-    }
+    if (Positioned.debugId == id) ???
     myUniqueId = id
   }
 

docs/docs/internals/debug-macros.md

@@ -0,0 +1,33 @@
+---
+layout: doc-page
+title: "Debug Macros"
+---
+
+Complex macros may break invariants of the compiler, which leads to compiler crashes.
+Here we lists common compiler crashes and how to deal with them.
+
+## position not set
+
+For this problem, here is the log that is usually shown:
+
+```
+[error] assertion failed: position not set for org.scalactic.anyvals.PosZInt.+$extension3(SizeParam.this.minSize)(
+[error]   org.scalactic.anyvals.PosZInt.widenToInt(SizeParam.this.sizeRange)
+[error] ) # 2326942 of class dotty.tools.dotc.ast.Trees$Apply in library/src-bootstrapped/scala/tasty/reflect/utils/TreeUtils.scala
+```
+
+To debug why the position is not set, note the tree id `2326942`, and enable
+the following compiler option:
+
+```
+-Ydebug-tree-with-id 2326942
+```
+
+With the option above, the compiler will crash when the tree is created. From
+the stack trace, we will be able to figure out where the tree is created.
+
+If the position is in the compiler, then either report a compiler bug or
+fix the problem with `.withSpan(tree.span)`. The following fix is an example:
+
+- https://github.com/lampepfl/dotty/pull/6581
+

docs/sidebar.yml

@@ -183,6 +183,8 @@ sidebar:
           url: docs/internals/type-system.html
         - title: "Dotty Internals 1: Trees & Symbols (Meeting Notes)"
           url: docs/internals/dotty-internals-1-notes.html
+        - title: Debug Macros
+          url: docs/internals/debug-macros.html
     - title: Resources
       subsection:
         - title: Talks